Method for separating a synthesis gas

ABSTRACT

A method for separating a synthesis gas containing carbon monoxide and hydrogen including compressing a flow of synthesis gas received from a source of synthesis gas in a compressor, purifying the compressed synthesis gas in a purification unit to purify it of water and/or carbon dioxide, cooling the compressed and purified flow of synthesis gas, separating the cooled flow of synthesis gas by washing and/or distillation at a cryogenic temperature and optionally by adsorption in a separating unit, and producing at least the following three gases in the separating unit: a carbon monoxide-enriched gas, a hydrogen-enriched gas, a residual gas containing carbon monoxide and hydrogen that is less pure with respect to carbon monoxide than the carbon monoxide-enriched gas and less pure with respect to hydrogen than the hydrogen-enriched gas.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a 371 of International PCT Application PCT/FR2017/051841, filed Jul. 6, 2017, which claims priority to French Patent Application 1657108, filed Jul. 25, 2016, the entire contents of which are incorporated herein by reference.

BACKGROUND

The present invention relates to a method for separating a synthesis gas. In particular, the synthesis gas comprises carbon monoxide and hydrogen and optionally methane and/or nitrogen and/or argon.

A cryogenic distillation apparatus is supplied with synthesis gas by a synthesis gas generating unit of the reforming and/or partial oxidation type (SMR, ATR, POX, etc.). The synthesis gas leaving the generator is cooled (for producing steam and various preheating steps and optionally with cooling water), then sent to a system for extracting CO₂ (typically by washing with amines such as MEA, MDEA, aMDEA), and then dried before being sent to the cryogenic distillation apparatus for separation and purification of CO. In certain cases, notably when the synthesis gas is produced by a unit for generation by partial oxidation producing acetylene, the residual gas called AOG or “acetylene offgas” constitutes the synthesis gas. It is compressed with a compressor upstream of the cryogenic distillation apparatus. The pressure to which this gas is compressed is compatible with the required pressure of hydrogen-rich product. Thus, it is not obligatory to recompress hydrogen prior to its transfer to the customer.

Generally, when the unit that consumes the CO is being operated at reduced load, the synthesis gas generating part only delivers the required CO molecules (so as not to increase the operating costs and as far as possible avoid torching the surplus products CO and/or H₂). The cryogenic distillation apparatus will operate at a low level, expressed as the ratio of the flow of CO and/or of H₂ to the nominal feed flow.

Thus, the operational stability of the cryogenic distillation apparatus at low load over the long term poses a problem with increased risks of not meeting the required purity of the CO or of having production flow rates that fluctuate excessively.

To ensure stable operation of the cryogenic distillation apparatus, it is necessary to operate it with a sufficient feed rate, which may be greater than the customers' requirements at any given time. The operating costs are degraded through having to operate the cryogenic distillation apparatus at a feed level exceeding customers' needs.

When the synthesis gas generating unit stops suddenly, this leads to stoppage of the cryogenic distillation apparatus, which is no longer supplied with molecules of synthesis gas. Once the synthesis gas becomes available again, a certain length of time (of the order of several hours) is required for restarting before being able to produce CO again at the required purity.

Thus, stoppage of the synthesis gas generating unit for some minutes causes shutdown of CO production for several hours. Therefore, in their turn, the customers downstream will have to stop their CO-consuming installations, which in their turn will have to be restarted.

SUMMARY

The technical solution presented hereunder therefore makes it possible to prevent long stoppages of an industrial installation causing increased costs through nonproduction: recycling of the fluids produced by the cryogenic distillation apparatus to the inlet of this apparatus (operating for example by washing with methane and/or partial condensation and/or washing with carbon monoxide) during operation at reduced load or during stoppage of the unit that generates the synthesis gas.

A cryogenic distillation apparatus and an adsorption unit are generally used in the production of carbon monoxide and hydrogen from synthesis gas generated by reforming or partial oxidation. The cryogenic distillation apparatus ensures purification of the CO and production of a hydrogen-enriched flow, which is then purified of hydrogen by adsorption.

The proposed solution is to recycle gases produced by the cryogenic distillation apparatus, optionally all the gases produced by the cryogenic distillation apparatus, upstream of the cryogenic distillation apparatus and of the synthesis gas compressor. Thus, the separating units and the compressor are not stopped, and when the synthesis gas generating source resumes operation, it again becomes possible to process the synthesis gas and produce the required products.

This solution is also able to ensure decreases in feed down to 0% from the source without stopping instead of typically 40%.

DE-A-102007059543 describes a method in which in the case of a reduction in feed of synthesis gas, products from a gas separation process are recycled to the feed, each being compressed by a dedicated compressor.

The present invention reduces the total cost of the apparatus, while eliminating the need to stop the synthesis gas compressor and start the product compressors.

According to one aspect of the invention, a method is provided for separating a synthesis gas containing carbon monoxide and hydrogen in which:

i) a flow of synthesis gas from a source of synthesis gas is compressed in a compressor

ii) the compressed synthesis gas is purified in a purification unit to purify it of water and carbon dioxide

iii) the compressed and purified flow of synthesis gas is cooled

iv) the flow of cooled synthesis gas is separated by washing and/or distillation at a cryogenic temperature and optionally by adsorption in a separating unit and

v) at least the following three gases are produced in the separating unit: a carbon monoxide-enriched gas, a hydrogen-enriched gas, a residual gas containing carbon monoxide and hydrogen that is less pure with respect to carbon monoxide than the carbon monoxide-enriched gas and less pure with respect to hydrogen than the hydrogen-enriched gas and optionally also a methane-enriched gas, and/or a nitrogen-enriched gas, and

vi) only if the flow of synthesis gas received from the source and sent to the compressor is below a threshold or zero, at least one part of each of the following three gases: the carbon monoxide-enriched gas, the hydrogen-enriched gas and the residual gas containing carbon monoxide and hydrogen is sent downstream of the source, to be purified in the purification unit and separated in the separating unit, characterized in that the at least one part of the three gases is sent upstream of the compressor to be compressed in the compressor upstream of the purification.

According to another aspect of the invention, in the method:

a) the compressed synthesis gas is stored while the flow of synthesis gas received from the source is above the threshold and

b) at least one part of the compressed synthesis gas previously stored as well as at least one part of at least each of the first three gases in step v) downstream of the source, and optionally upstream of the compressor, are sent to be purified in the purification unit and separated in the separating unit.

According to other optional aspects of the invention:

-   -   at least one part of at least each of the first three gases in         step v) downstream of the source and optionally upstream of the         compressor only if the flow of synthesis gas received from the         source is below the threshold, or even if no flow is received         from the source.

only x% of the carbon monoxide-enriched gas, y% of the hydrogen-enriched gas and z% of the residual gas are sent to the separating unit when the flow of synthesis gas received from the source is below a threshold, x, y and z differing by at most 5%.

-   -   part of the carbon monoxide-enriched gas is stored in a storage         facility,

and the rest of the carbon monoxide-enriched gas is sent to a customer if the flow of synthesis gas received from the source is above the threshold and carbon monoxide-enriched gas received from the storage facility is sent to the customer if the flow of synthesis gas received from the source is below the threshold.

-   -   if the flow of synthesis gas received from the source is below         the threshold, all the carbon monoxide-enriched gas sent to the         customer comes from the storage facility and optionally all the         carbon monoxide-enriched gas received directly from the         separating unit without passing through the storage facility is         sent upstream of the compressor and downstream of the source.     -   the synthesis gas received from the source is a residual gas         from an acetylene production unit.     -   if the flow of synthesis gas received from the source is above         the threshold, part of the synthesis gas is stored in a storage         facility and if the flow of synthesis gas is below the         threshold, synthesis gas is sent from the storage facility to         the compressor.     -   if the flow of synthesis gas received from the source is zero,         the separating unit is only supplied with gases produced by the         separating unit.     -   the carbon monoxide-enriched gas is compressed in a product         compressor and is sent to the customer and if the flow of         synthesis gas received from the source is below the threshold         and the flow of carbon monoxide-enriched gas to be compressed is         below another threshold, gaseous nitrogen is sent to the product         compressor to be compressed with the carbon monoxide-enriched         gas.     -   if the flow of synthesis gas received from the source is below a         threshold, a gas rich in a component of the synthesis gas         received from another source that is not the separating         apparatus is sent to the separating unit.     -   the carbon monoxide-enriched gas is compressed in a compressor,         and then split into two, one compressed part serving as a         product and one compressed part being recycled downstream of the         source and/or the hydrogen-enriched gas is compressed in a         compressor, and then split into two, one compressed part serving         as a product and one compressed part being recycled downstream         of the source.     -   only if the flow of synthesis gas received from the source and         sent to the compressor is below a threshold or zero, all the         carbon monoxide-enriched gas is sent downstream of the source         and upstream of the compressor.     -   only if the flow of synthesis gas received from the source and         sent to the compressor is below a threshold or zero, all the         hydrogen-enriched gas is sent downstream of the source and         upstream of the compressor.     -   only if the flow of synthesis gas received from the source and         sent to the compressor is below a threshold or zero, all the         residual gas containing carbon monoxide and hydrogen is sent         downstream of the source, and upstream of the compressor.     -   the residual gas contains methane and optionally nitrogen and/or         argon.     -   the total percentage of hydrogen and carbon monoxide in the         residual gas is below 98 mol %     -   the at least one parts of each of the following three gases: the         carbon monoxide-enriched gas, the hydrogen-enriched gas and the         residual gas containing carbon monoxide and hydrogen are only         compressed in the compressor.

In particular, in the case when the synthesis gas is compressed upstream of the cryogenic distillation apparatus by a compressor (for example, such as when the synthesis gas consists of a stream of residual gas (called “Acetylene Offgas”) from a unit producing acetylene), the synthesis gas compressor is used for recycling the fluids produced by the cryogenic distillation apparatus in order to:

1) Maintain a sufficient flow to the cryogenic distillation apparatus to supplement the flow of synthesis gas produced by the upstream unit and to be able to produce a flow of CO less than the technical minimum of the cryogenic distillation apparatus.

2) Maintain the cryogenic distillation apparatus in operation including during stoppage of the unit producing synthesis gas, in this case the cryogenic distillation apparatus will only be supplied with the recycled gases, until the synthesis gas produced by the upstream unit is available again.

-   -   To ensure that the customer is supplied with CO during the time         that the synthesis gas is not available, a liquid storage         facility for CO molecules will be added to the cryogenic         distillation apparatus. This CO will be vaporized throughout the         interruption of supply from the cryogenic distillation         apparatus.     -   To compensate the starting time for vaporization, a buffer tank         for gaseous CO can be added. In fact, the gases produced by the         cryogenic distillation apparatus will then be recycled and the         gas required by the customer will then come from this buffer         tank installed upstream of the control valve on the CO         production line and will be operated at a pressure above the         pressure required by the CO consuming unit.     -   To compensate the opening time of the valves of the recycling         pipelines, a high-pressure buffer tank can be added, which will         be fed by the outlet of the AOG compressor and will be connected         to the intake of the compressor. This buffer tank will therefore         be able to compensate the first seconds of the transient case of         closure of the valve as well as the very slight leaks at the         level of the compressor and/or of the CO₂ extraction system.     -   Nitrogen could optionally be injected at the intake of the CO         compressor to balance the leaks at the level of the compressor.         However, it will be preferable to compensate the losses of the         CO compressor by supplementing with external molecules from a         gaseous CO buffer tank of the liquid CO tank.

In fact, adding nitrogen degrades the quality of the CO produced.

These ideas, taken together, mean it is always possible to maintain a flow of CO production to the customer including during brief stoppage of the synthesis gas generating unit and thus safeguard the installation for the end customer by smoothing the interruptions of supply of the installation for generating synthesis gas.

BRIEF DESCRIPTION OF THE DRAWINGS

For a further understanding of the nature and objects for the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, in which like elements are given the same or analogous reference numbers and wherein:

FIG. 1 illustrates a schematic representation in accordance with one embodiment of the present invention.

FIG. 2 illustrates a schematic representation in accordance with one embodiment of the present invention; and

FIG. 3 illustrates a schematic representation in accordance with one embodiment of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

In FIG. 1, a synthesis gas 1 is received from a synthesis gas generating unit, for example a reforming unit, such as an SMR or an ATR or a partial oxidation unit, which for example produces acetylene. The synthesis gas 1 contains carbon monoxide, carbon dioxide, hydrogen and optionally methane and nitrogen.

The gas 1 is compressed by a compressor 3 to form a compressed gas 5. The compressed gas 5 is sent to a hydrogenation treatment unit 7 for removing the oxygen and the unsaturated hydrocarbons, producing a flow 9. The flow 9 is sent to a CO₂ purification unit 13, for example by washing with amines, producing a purge flow rich in CO₂ 11 and a flow of synthesis gas depleted of CO₂ 15. The flow 15 is sent to a unit for CO₂ purification by adsorption 17 for removing the remaining CO₂. The synthesis gas purified of CO₂ 19 is sent to a cryogenic distillation unit 21, where the synthesis gas is cooled and then separated in at least one distillation column, comprising for example a step of washing with methane and/or partial condensation and/or washing with carbon monoxide.

The cryogenic distillation unit 21 produces a carbon monoxide-enriched gaseous flow 26, a hydrogen-rich gas 27 and a residual gas 23 containing carbon monoxide and hydrogen, less pure with respect to carbon monoxide than the gas 26 and less pure with respect to hydrogen than the gas 27. The residual gas 23 preferably contains methane and optionally nitrogen and/or argon. Preferably the total percentage of hydrogen and carbon monoxide in the residual gas is below 98 mol %.

The carbon monoxide-enriched gaseous flow 26 is compressed in a compressor 33. A gaseous flow 31 received from the outlet of the compressor 33 is returned to the cryogenic distillation unit 21. The flow 37 represents losses of the carbon monoxide-enriched gases from the compressor 33. The gas from an intermediate stage of the compressor 33 forms a product in part 39.

If the flow of synthesis gas received from the source of synthesis gas is zero, the whole of these three products 23, 27, 39 is returned to purification. Hydrogen and the residual gas must be compressed in compressor 3 but the carbon monoxide may be compressed to the required pressure in compressor 33 and recycled downstream of the compressor 3. Thus, gases 23 and 27 must be sent upstream of compressor 3 whereas gas 26 may be recycled upstream or downstream of compressor 3.

If the flow of synthesis gas received from the source is not zero but is below a threshold, a percentage of x% of the carbon monoxide-rich gas, y% of the hydrogen-rich gas and z% of the residual gas may be returned to the purification unit and then to cryogenic separation. The percentages x, y and z are preferably identical but may differ by less than 5%.

When at least one part of the carbon monoxide from compressor 33 is recycled, gaseous nitrogen 35 may be injected at an intermediate level or at the intake of compressor 33 to compensate the leaks from the compressor.

FIG. 2 differs from FIG. 1 in that the hydrogen-enriched flow 30 is sent from the adsorption unit 17, which removes the water and the CO₂, to an apparatus for purification by adsorption 41. This apparatus 41 produces a hydrogen-rich gas 43.

The gas depleted of hydrogen 47 received from apparatus 41 is mixed with the residual product 49 from apparatus 21 and compressed in a compressor 53 to supply a residual gas under pressure.

Thus, if the synthesis gas 1 from the source is no longer produced or is produced in reduced amount, at least some products from the cryogenic distillation apparatus 21 are recycled upstream of the compressor 3, optionally after an additional treatment step. In addition to the rest 51 of the gas from the intermediate stage of the compressor 33 or in place of gas 51, at least one part 45 of the hydrogen-rich product 43 and/or at least one part 55 of the residual gas received from the compressor 53 may be recycled upstream of the compressor 3.

The residual gas 55 preferably contains methane and optionally nitrogen and/or argon. Preferably the total percentage of hydrogen and carbon monoxide in the residual gas is below 98 mol %.

FIG. 3 differs from FIG. 1 in that a part 4 of the synthesis gas is taken at an intermediate pressure of the compressor 3 for part 4 and at the final pressure of the latter for part 4 and are not treated in units 7, 13, 17, 21. The part 4 is sent from the final pressure of the compressor 3 to an adsorption separating unit called PSA 16 where it is purified to form a hydrogen-rich flow 52. The residual product 18 from the adsorption separating unit 16 depleted of hydrogen is mixed with the residual gas 49 from the cryogenic apparatus 21 and the mixture is compressed in a compressor 22 to form a gaseous fuel 20. The part 12 of synthesis gas may be mixed with the gaseous fuel 20.

If the synthesis gas 1 received from the source is no longer produced or is produced in reduced amount, below a threshold, at least the following three gases are recycled, downstream of the source of the synthesis gas and optionally upstream of the compressor 3:

i) At least one part 51 of the carbon monoxide-enriched gas 39 received from the cryogenic distillation apparatus 21.

ii) At least one part 45 of the hydrogen-enriched gas received from the adsorption separating unit 16.

iii) At least one part 14 of a residual gas containing hydrogen and/or carbon monoxide received from the cryogenic distillation apparatus and/or adsorption separating unit 16, less pure with respect to carbon monoxide than gas 51 and less pure with respect to hydrogen than gas 45, and optionally also

iv) A part 6 of the compressed synthesis gas taken downstream of the compressor 3, stored in a storage facility 8 while the flow of synthesis gas is above the threshold and returned via pipe 10 and a valve.

The residual gas 14 preferably contains methane and optionally nitrogen and/or argon. Preferably the total percentage of hydrogen and carbon monoxide in the residual gas is below 98 mol %.

Preferably the percentages of the recycled gases differ little, for reconstituting the synthesis gas by mixing the different products 45, 51, 14.

To ensure that there is sufficient carbon monoxide for the customer, it is possible to use a liquid carbon monoxide tank 24 within the distillation apparatus 21 and/or a gaseous carbon monoxide tank 38 fed from the compressor 33 via pipe 36 for supplying the carbon monoxide that is lacking. The liquid from storage facility 24 is vaporized in the vaporizer 25.

In particular, if a large part or all of the carbon monoxide produced in real time is destined for recycling, the carbon monoxide stored in tanks 24 or 38 may make it possible to supply the customer. Thus, for example, valve 40 will be opened to supply carbon monoxide from the storage facility 38 to the customer if there will be a shortage of synthesis gas 1.

In a preferred variant, which may be used for all the methods in FIGS. 1 to 3 using pipelines that are present or by adding the necessary storage facilities, recycling is carried out as follows.

Firstly, the flow of synthesis gas falls below a threshold or stops. First of all, we begin to open recycling valves for at least two fluids produced by treating the synthesis gas.

The synthesis gas is recycled from the storage facility 8 (or from the pipe downstream of compressor 3 if its volume and its pressure allow this) to upstream of compressor 3.

Once the recycling valves are open, at least the following three gases are recycled, downstream of the source of the synthesis gas and optionally upstream of compressor 3:

i) At least one part 51 of the carbon monoxide-enriched gas 39 received from the cryogenic distillation apparatus 21.

ii) At least one part 45 of the hydrogen-enriched gas received from the adsorption separating unit 16.

iii) At least one part 14 of a residual gas containing hydrogen and/or carbon monoxide received from the cryogenic distillation apparatus and/or adsorption separating unit 16, less pure with respect to carbon monoxide than gas 51 and less pure with respect to hydrogen than gas 45, and optionally also

iv) A part 6 of the compressed synthesis gas taken downstream of compressor 3, stored in a storage facility 8 while the flow of synthesis gas is above the threshold and returned via pipe 10 and a valve.

In certain cases, the entire production of the separating apparatus consisting of the separating apparatus by washing and/or distillation and optionally by the adsorption unit is recycled upstream of the synthesis gas compressor.

It is also possible, for all the figures, to recycle the carbon monoxide-enriched gas 51 downstream of the synthesis gas compressor 3. The gas 51 may in this case be taken at the outlet pressure of compressor 3.

It is also possible to recycle at least two products and also send a gas 2 rich in a component of the synthesis gas obtained from an external source, other than the separating apparatus, upstream or downstream of the synthesis gas compressor. This gas 2 may be natural gas obtained from a pipeline or a synthesis gas from another source.

It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above. 

1.-15. (canceled)
 16. A method for separating a synthesis gas containing carbon monoxide and hydrogen comprising: i) compressing a flow of synthesis gas received from a source of synthesis gas in a compressor, ii) purifying the compressed synthesis gas in a purification unit to purify it of water and/or carbon dioxide, iii) cooling the compressed and purified flow of synthesis gas, iv) separating the cooled flow of synthesis gas by washing and/or distillation at a cryogenic temperature and optionally by adsorption in a separating unit, and v) producing at least the following three gases in the separating unit: a carbon monoxide-enriched gas, a hydrogen-enriched gas, a residual gas containing carbon monoxide and hydrogen that is less pure with respect to carbon monoxide than the carbon monoxide-enriched gas and less pure with respect to hydrogen than the hydrogen-enriched gas and optionally also a methane-enriched gas, and/or a nitrogen-enriched gas, and vi) sending at least one part of each of the following gases downstream of the source, only if the flow of synthesis gas received from the source and sent to the compressor is below a threshold or zero, the carbon monoxide-enriched gas, the hydrogen-enriched gas, and the residual gas containing carbon monoxide and hydrogen, to be purified in the purification unit and separated in the separating unit, wherein the at least one part of the three gases is sent upstream of the compressor to be compressed in the compressor upstream of the purification.
 17. The method as claimed in claim 16, further comprising: a) storing compressed synthesis gas while the flow of synthesis gas received from the source is above the threshold, and b) sending at least one part of the compressed synthesis gas, previously stored, as well as at least one part of at least each of the first three gases in step v), downstream of the source.
 18. The method as claimed in claim 16, wherein at least one part of at least each of the three gases in step v) downstream of the source and upstream of the compressor only if the flow of synthesis gas received from the source is below the threshold, or even if no flow is received from the source.
 19. The method as claimed claim 16, wherein only x% of the carbon monoxide-enriched gas, y% of the hydrogen-enriched gas and z% of the residual gas are sent to the separating unit when the flow of synthesis gas received from the source is below a threshold, x, y and z differing by at most 5%.
 20. The method as claimed in claim 16, wherein a part of the carbon monoxide-enriched gas is stored in a storage facility and the rest of the carbon monoxide-enriched gas is sent to a customer if the flow of synthesis gas received from the source is above the threshold and the carbon monoxide-enriched gas received from the storage facility is sent to the customer if the flow of synthesis gas is below the threshold.
 21. The method as claimed in claim 20, wherein if the flow of synthesis gas received from the source is below the threshold, all the carbon monoxide-enriched gas sent to the customer comes from the storage facility is sent upstream of the compressor and downstream of the source.
 22. The method as claimed in claim 16, wherein the synthesis gas is a residual gas from an acetylene production unit.
 23. The method as claimed in claim 16, wherein if the flow of synthesis gas received from the source is above the threshold, part of the synthesis gas is stored in a storage facility and if the flow of synthesis gas received from the source is below the threshold, synthesis gas is sent from the storage facility to the compressor.
 24. The method as claimed in claim 16, wherein if the flow of synthesis gas received from the source is zero, the separating unit is only supplied with gases produced by the separating unit.
 25. The method as claimed in claim 16, wherein the carbon monoxide-enriched gas is compressed in a product compressor and is sent to the customer and if the flow of synthesis gas received from the source is below the threshold and the flow of carbon monoxide-enriched gas to be compressed is below another threshold, gaseous nitrogen is sent to the product compressor to be compressed with the carbon monoxide-enriched gas.
 26. The method as claimed in claim 16, wherein if the flow of synthesis gas received from the source is below a threshold, a gas rich in a component of the synthesis gas received from another source, which is not the separating unit, is sent to the separating unit.
 27. The method as claimed in claim 16, wherein the carbon monoxide-enriched gas is compressed in a product compressor, then split into two, one compressed part serving as a product and one compressed part being recycled downstream of the source and/or the hydrogen-enriched gas is compressed in a product compressor, then split into two, one compressed part serving as a product and one compressed part being recycled downstream of the source.
 28. The method as claimed in claim 16, wherein the residual gas contains methane.
 29. The method as claimed in claim 16, wherein the total percentage of hydrogen and carbon monoxide in the residual gas is below 98 mol %.
 30. The method as claimed in claim 16, wherein the at least one part of each of the following three gases: the carbon monoxide-enriched gas, the hydrogen-enriched gas and the residual gas containing carbon monoxide and hydrogen are only compressed in the compressor. 